Fix #6914: Improve type inference for implicit Conversions

Make use of the knowlegde that a given instance used for a conversion
must be an instance of `scala.Conversion`.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -1367,11 +1367,21 @@ trait Implicits { self: Typer =>
         if (argument.isEmpty)
           adapt(generated, pt.widenExpr, locked)
         else {
-          val untpdGenerated = untpd.TypedSplice(generated)
-          def tryConversion(implicit ctx: Context) =
+          def untpdGenerated = untpd.TypedSplice(generated)
+          def tryConversion(implicit ctx: Context) = {
+            val untpdConv =
+              if (ref.symbol.is(Given))
+                untpd.Select(
+                  untpd.TypedSplice(
+                    adapt(generated,
+                      defn.ConversionClass.typeRef.appliedTo(argument.tpe.widen, pt),
+                      locked)),
+                  nme.apply)
+              else untpdGenerated
             typed(
-              untpd.Apply(untpdGenerated, untpd.TypedSplice(argument) :: Nil),
+              untpd.Apply(untpdConv, untpd.TypedSplice(argument) :: Nil),
               pt, locked)
+          }
           if (cand.isExtension) {
             val SelectionProto(name: TermName, mbrType, _, _) = pt
             val result = extMethodApply(untpd.Select(untpdGenerated, name), argument, mbrType)

tests/pos/i6914.scala

@@ -0,0 +1,19 @@
+trait Expr[T]
+trait Liftable[T]
+
+/*given autoToExpr[T] as Conversion[T, Expr[T]] given Liftable[T] {
+  def apply(x: T): Expr[T] = ???
+}
+*/
+class ToExpr[T] given Liftable[T] extends Conversion[T, Expr[T]] {
+  def apply(x: T): Expr[T] = ???
+}
+given toExpr[T] as ToExpr[T] given Liftable[T]
+
+given as Liftable[Int] = ???
+given as Liftable[String] = ???
+
+def x = the[ToExpr[String]]
+def y = the[Conversion[String, Expr[String]]]
+
+def a: Expr[String] = "abc"